Synthetic polymers, particularly solution polymerized polymers made by anionic polymerization techniques, have been employed in the manufacture of tires. The handling of these polymers prior to tire manufacture can be difficult because the polymers tend to flow under ambient conditions. This flow has been referred to as cold flow.
To alleviate cold flow problems, the polymers can be coupled. For example, living polymers can be reacted with multi-valent compounds such as tin tetrachloride or silicon tetrachloride to provide coupled polymers that are less susceptible to cold flow. This reduction in the tendency to flow is believed to be caused by a viscosity increase that occurs when the polymers are coupled. While this increase in viscosity is advantageous for cold flow, it can be undesirable for processing. That is, highly viscous polymers can be difficult to mix with other ingredients that are often included in rubber formulations such as fillers.
Advantageously, certain coupling agents provide bonds to the polymer chains that are believed to break under mixing conditions. For example, the carbon-tin bond within polymers coupled with tin tetrachloride are believed to break during mixing as evidenced by a decrease in sheer viscosity at mixing conditions.
While polymer coupling has proven to be technologically useful, the coupling of the polymer results in lost opportunity for other advantageous reactions with the polymer live ends. As is known in the art, polymer live ends can be reacted with functionalizing agents that provide benefit to the rubber compositions. For example, living polymers have been reacted with compounds such as tributyltin chloride or 1,3-dimethyl-2-imidazolidinone to provide functional groups that have desirable impact in filled rubber vulcanizates. Indeed, functional groups deriving from reactions with these compounds are believed to reduce the rolling resistance of tire treads prepared with polymers bearing these functional groups.
Since these functional groups offer little or no cold flow reduction, coupling agents and functionalizing agents may be used together in an effort to achieve a desirable balance of properties. While this has proven to be technologically useful, drawbacks exist. Namely, neither maximum cold flow reduction nor maximum polymer functionalization can be achieved.
There is therefore a need for an advancement in this technology.